Interrater reliability of the multiple sleep latency test

The multiple sleep latency test

The measurement of daytime sleepiness is important in the evaluation of patients with excessive sleepiness. The multiple sleep latency test (MSLT) is an objective test that measures the tendency to fall asleep under controlled conditions. It is based on the notion that sleep latency reflects underlying physiological sleepiness. The MSLT consists of four to five naps given 2h apart during the day, following a standardized procedure. The mean sleep latency from all naps is used as the measure of sleepiness. The test has been shown to be valid and reliable and is part of the diagnostic criteria for narcolepsy and idiopathic hypersomnia. However, the MSLT is affected by numerous variables including insufficient sleep, drugs, activity, and arousal level. Adherence to the established protocol is necessary to limit the effect of these extraneous factors on the MSLT. While the test is a valuable and widely used diagnostic tool for narcolepsy and idiopathic hypersomnia, the use of MSLT in other sleep disorders is not well established.

Enhancements to the multiple sleep latency test

INTRODUCTION

The utility of multiple sleep latency tests (MSLTs) is limited to determining sleep onset latency (SOL) and rapid eye movement sleep latency. The odds ratio product (ORP) is a continuous index of sleep depth with values of 0, 1.0, and 2.5 reflecting very deep sleep, light sleep, and full wakefulness, respectively. We determined the time course of sleep depth during MSLT naps expecting that this would enhance the test's clinical utility.

METHODS

Thirty MSLTs (150 naps) were performed for excessive somnolence. Patients indicated whether they slept (yes/no) after each nap. SOL was scored by two experienced technologists. Time course of ORP was determined with a commercial system. We determined ORP at SOL (ORPSOL), times ORP decreased <2.0, <1.5, <1.0 and <0.5 during the entire nap duration, and the integral of decrease in ORP over nap duration (ΔORPINT).

RESULTS

SOL occurred almost invariably when ORP was between 1.0 and 2.0. Of 47 naps (21 patients) with SOL <5 minutes, ORP decreased <1.0 (light sleep) in <5 minutes in only 13 naps (nine patients) and <0.5 (deep sleep) in only two naps in one patient. The relation between ORPINT and frequency of sleep perception was well defined, allowing determination of a threshold for sleep perception. This threshold ranged widely (5-50 ΔORP*epoch).

CONCLUSION

As currently identified, SOL reflects transition into a highly unstable state between wakefulness and sleep. Reporting the times of attaining different sleep depths may help better identify patients at high risk of vigilance loss. Furthermore, an ORPSOL outside the range 1.0-2.0 can help identify scoring errors.

Objective measures of sleepiness and wakefulness: application to the real world?

The impact of excessive sleepiness on the individual and on society is immense, and chronic sleepiness is one of the most common complaints evaluated by sleep medicine specialists. The author explores how measures of sleepiness and wakefulness using the Multiple Sleep Latency Test (MSLT) and the Maintenance of Wakefulness Test (MWT) can be clinically useful. A companion article presents a counterpoint discussion of the limitations and drawbacks associated with the MSLT and MWT. Both presentations use an evidence-based approach to understanding the operating characteristics of these tests, and the overall goal is to clarify for sleep medicine specialists the positive and negative attributes of the MSLT and MWT. The MSLT and MWT are the most widely accepted objective measures of an individual's ability to fall asleep and ability to remain awake, respectively. The MSLT is a well-validated and extensively published objective measure of the speed at which a subject falls asleep under standardized laboratory conditions, and it is associated with good to excellent interrater and intrarater reliability, and excellent test-retest reliability. The MSLT is indicated as part of the evaluation of suspected narcolepsy and it may be helpful in differentiating narcolepsy from idiopathic hypersomnia. Mean sleep latency values less than 5 minutes are observed in the majority of subjects with narcolepsy, and the presence of two or more sleep-onset rapid eye movement periods is strongly correlated with a diagnosis of narcolepsy. An MSLT should be performed to address specific clinical questions, and should not be used as a screening tool. The MWT has clinical usefulness in evaluating response to treatment following intervention for conditions associated with excessive sleepiness, and in assessing individuals who must remain awake for safety reasons. However, the sleep medicine specialist should not rely solely on mean sleep latency values as a single indicator of impairment or risk of accidents, but findings should be integrated with the clinical history, compliance, patient judgment, and other factors to form a global impression regarding the individual's response to treatment. Future challenges include refinement of normative ranges in different populations using rigorous statistical methods, and improved understanding of the specific operating characteristics of the MSLT and MWT in different age groups. Additional study is necessary regarding the impact of MSLT and MWT findings on clinical decision-making, patient outcome, and patient and physician satisfaction. From a safety and regulatory standpoint, additional study is needed to establish the correlation between MWT findings and the risk of adverse consequences of sleepiness such as accidents. Because the MSLT and MWT are in-laboratory tests, it is important that investigators also develop novel techniques that provide reliable assessment of sleepiness and wakefulness in the actual work environment over extended periods. In summary, the MSLT and MWT are not perfect tests, but they are the best objective measures currently available for characterization of ability to fall asleep and ability to remain awake.

ACNS Clinical Controversy: MSLT and MWT Have Limited Clinical Utility

The Multiple Sleep Latency Test (MSLT) and the Maintenance of Wakefulness Test (MWT) are two commonly used laboratory-based objective tests to measure sleepiness and alertness, respectively. Data suggest both are extremely sensitive tests when measuring the effects of sleep deprivation within subjects, but are less sensitive for confirming sleepiness and response to treatment in groups of patients with different sleep disorders. Inconsistent and even sometimes paradoxical test results may be partly explained by data that show the MSLT and MWT are not selectively sensitive to either sleepiness or alertness, but sensitive to both the sleep and the arousal systems. Sleep latencies seen on both the MSLT and MWT are affected to varying degrees by a myriad of internal and external influences that can alter what we would prefer each test to show. If we continue to use these tests to measure sleepiness or alertness in patients with different sleep disorders, we need to understand more about the nature and impact of different sources of internal and external arousal so that we can better control the test environment. Improved understanding of the determinants of sleep onset is essential because excessive sleepiness has important consequences for both individuals and society.

Daytime sleepiness and its evaluation

Basic models of sleepiness, focusing on the homeostatic and circadian components of sleepiness, are able to predict important fluctuations of sleepiness. However, they fail in explaining certain sleepiness phenomena, as for instance in insomnia patients. To meet this shortcoming, modern models incorporate the arousal component of sleepiness, in addition to the sleep drive. While these models mainly concentrate on short-term changes in sleepiness, "state" sleepiness, there are indications that a stable characteristic level of sleepiness, "trait" sleepiness, is also an important determinant of a person's level of sleepiness. This leads to a conceptualization of sleepiness in which situational factors modify a basal level of sleep drive and arousal. It implies that sleepiness is not a unitary concept and can reflect essentially different states. Multiple sleepiness assessment tools have been proposed in the past. The majority of them offer valuable information, but they do not grasp all aspects of sleepiness. We should bear in mind that tools for assessing sleepiness are always operationalizations reflecting the theoretical framework the investigator has on sleepiness. Hence, rather than searching for a gold standard for the measurement of sleepiness, future research effort should be aimed at linking the various measurement techniques with the hypothesized underlying components of sleepiness on a sound empirical basis.